Controls on Spatial and Temporal Variation in Snow Accumulation on Glaciers in the Southern Alps, New Zealand

Abstract:

Mountain glaciers are already responding to climatic warming, and are expected to
make a substantial contribution to sea-level rise in the coming decades. The aim of
this investigation in the New Zealand Southern Alps was to improve our
understanding of snow accumulation variability on mid-latitude maritime glaciers,
in order to allow for better estimation of future glacier mass balance. The specific
aim was to investigate snow accumulation processes at a range of spatial and
temporal scales, focussing on synoptic-scale atmospheric circulation influences,
moisture sources for snow accumulation and local-scale dependencies of snow
accumulation in relation to topography. A range of methods were utilised including
direct measurement, snow and ice core analysis, statistical analysis and modelling.
Snow accumulation in the Southern Alps was found to be derived predominantly
from the Tasman Sea, and deposited during low pressure troughs and fronts.
Although precipitation increased with elevation, wind processes redistributed this
mass. On a ~monthly timescale this redistribution caused an unexpected result,
namely that wind deflation of snow on Franz Josef Glacier countered the effects of
greater accumulation, and total accumulation was similar at both Franz Josef and
Tasman Glaciers over this period. These processes make it challenging to simulate
snow accumulation patterns by simply extrapolating snowfall over an orographic
barrier from lowland climate station data. On an inter-annual basis, temperature,
especially during the ablation season, had most influence on net accumulation, and
warm summers served to homogenise winter variability. Consequently, atmospheric
circulation patterns that affect summer temperature, for example the El Niño
Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) also influence
inter-annual variability in net accumulation.
Together, these results highlight the dependence of maritime glaciers in the New
Zealand Southern Alps on the prevailing westerly circulation. Although some
uncertainty surrounds how global warming will affect atmospheric circulation and
synoptic weather patterns, the results of this research indicate that New Zealand
glaciers can be expected to lose significant mass in the coming decades if the
current positive trend in the SAM continues, and if La Niña events (positive ENSO)
become more frequent.